The present invention relates generally to drives for optically recording and reproducing an information signal using an optical recording medium such as an optical disk, and more particularly to a servo system for use in such a optical recording/reproducing drive which is arranged so as to improve the focusing servo and the tracking servo to allow higher recording/reproducing accuracy.
Generally, in recording/reproducing systems using an optical disk is known the so-called heat recording system where a laser beam spot having a diameter of about 1 .mu.m is formed on a recording film provided on a track of an optical disk so as to allow physical variation of the recording film due to local temperature increase to record the information signal. One conventional servo system will be described hereinbelow with reference to FIG. 1. In FIG. 1, illustrated at numeral 50 is an optical disk equipped with pregroups and illustrated at numeral 52 is a light source such as a laser light source. The light beam emitted from the light source 52 passes through a collimator lens 53 so as to be converted into a parallel light beam, which is in turn reflected by means of a beam splitter 54 and then condensed onto the optical disk 50 through an objective lens 55. On the other hand, the reflected light beam from the optical disk 50 passes through the objective lens 55 and further through the beam splitter 54 so as to be directed to photodetectors 57 and 58. The outputs of the photodetectors 57 and 58 are respectively inputted to a gain-variable differential amplifier 59 the output of which is supplied to an equalizer 60 and further to a drive amplifier 61. The output of the drive amplifier 61 is led to a tracking drive apparatus 56, which in turn moves the objective lens 55 radially with respect to the optical disk 50 to perform the tracking servo operation. In the tracking servo operation, in the case of recording the information signal on the optical disk 50, the output power of the laser light source 52 is increased and the output light of the laser light source 52 is modulated in accordance with the information signal, thereby effecting the head recording on the optical disk 50. At this time, since the power of the light incident on the tracking servo photodetectors 57 and 58 becomes increased, the gain of the tracking servo loop increases so as not to satisfy the optimal servo condition. Thus, the gain of the gain-variable differential amplifier 59 is required to be controlled by the output of an addition amplifier 63 so that the magnitude of the difference signal (a tracking signal) between the outputs of the photodectors 57 and 58, i.e., the output of the gain-variable differential amplifier 59, becomes constant. As the result of the gain control, the output of the gain-variable differential amplifier 59 takes a value to be obtained by dividing its input by the output of the addition amplifier 63 (standardization). Thus, the magnitude of the tracking error signal is controllable to substantially become constant irrespective of the exchange of the optical disk 50. Numeral 64 is a filter for performing the AGC control (the control of the gain-variable differential amplifier 59) with an optimal frequency characteristic.
Although as described above the conventional tracking servo system is arranged such that the gain-variable differential amplifier is used in order to make constant the servo loop gain of the optical disk to optimize the operation of the servo loop, there is a problem which arises with such an arrangement, however, in that the control precision of the AGC control can become insufficient when the gains and offsets of the addition amplifier 63 and gain-variable differential amplifier 59 vary because the AGC control loop cannot be constructed as a closed loop. In addition, the AGC control system is required to be also provided for the focusing servo system to thereby result in increase in the circuit amount and increase in the manufacturing cost.